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However, this factor alone cannot explain the recent warming nor, indeed, can it fully explain many past changes such as Earth’s ice ages. But what if changes in the Sun’s activity have larger-than-expected effects on the climate?

There are plenty of ideas about how this could happen. For instance, one as-yet-unproven idea is that changes in the relative amount of ultraviolet light emitted by the Sun might affect the ozone layer, heating the stratosphere and altering circulation patterns in the lower atmosphere.

Chilling idea

Increased sunspot activity is known to strengthen the Sun’s magnetic field, which deflects more of the galactic cosmic rays entering the solar system and thus reducing the number hitting Earth. The argument championed by Henrik Svensmark is that this would reduce cloud formation in the atmosphere – warming the Earth – and that this effect explains the recent global warming.

The case has been made at greater length in a book Svensmark wrote with science journalist Nigel Calder (who edited New Scientist from 1962 to 1966), called The Chilling Stars.

There are at least three separate issues here. First, do cosmic rays really trigger cloud formation? If so, how do the resulting changes in cloud cover affect temperature? Finally, does this explain the warming trend of the past few decades?

Far-fetched concept

There is no convincing evidence that cosmic rays are a major factor determining cloud cover. The ionising of air by cosmic rays will impart an electric charge to aerosols, which in theory could encourage them to clump together to form particles large enough for cloud droplets to form around, called “cloud condensation nuclei”.

But cloud physicists say it has yet to be shown that such clumping occurs. And even if it does, it seems far-fetched to expect any great effect on the amount of clouds in the atmosphere. Most of the atmosphere, even relatively clean marine air, has plenty of cloud condensation nuclei already.

A series of attempts by Svensmark to show an effect have come unstuck. Initially, Svensmark claimed there was a correlation between cosmic ray intensity and satellite measurements of total cloud cover since the 1980s – yet a correlation does not prove cause and effect. It could equally well reflect changes in solar irradiance, which inversely correlate with cosmic ray intensity.

Furthermore, this apparent correlation depended on adjustments to the data, and it does not hold up when more recent cloud measurements from 1996 onwards are included.

Beguiling fit

Svensmark and others then pointed to an apparent correlation between low-altitude cloud cover and cosmic rays. But after 1995, the beguiling fit of Svensmark’s graph depends on a “correction” of satellite data, and the satellite scientists say this is not justified. “It’s dubious manipulation of data in order to suit his hypothesis,” says Joanna Haigh, an atmospheric physicist at Imperial College London, UK.

Then there is the question of how changes in clouds will affect climate. Svensmark claims the overall effect of less cloud cover is a warmer world, with less heat loss due to reflection off clouds during the day outweighing higher loss of heat at night.

Yet even during the day, many clouds in the upper atmosphere can have a warming effect. Not all scientists agree that reducing cloud cover would warm the planet.

No trend

Finally, and most importantly, even if cosmic ray intensity does turn out to influence cloud cover and temperature, it cannot explain the warming trend of the past few decades. Direct measurements of cosmic ray intensity going back as far as 50 years show no downward trend coinciding with the recent warming.

Indirect measurements of cosmic rays, based on the abundance of certain atmospheric isotopes formed by them, suggest that intensity fell between 1900 and 1950. Yet while there can be a lag between a sudden jump in a climate “forcing” and its full effect on temperature, most warming should occur within a few years and taper off within decades..

The wild claims of Svensmark do not mean that the idea of a link between cosmic rays and clouds is nonsense. It is taken seriously by a small number of scientists. A handful of studies using different methods hint at a very tiny effect, though more have found none.

Experiments now underway at the European Laboratory for Particle Physics (CERN) should settle the issue of whether cosmic rays can trigger the formation of cloud condensation nuclei, though this will not reveal whether it matters in the real world.

The bottom line is that whether or not cosmic rays have affected the climate in the more distant past, they cannot explain our planet’s recent warming.